DESCRIPTION (provided by PI): Myoblast fusion is a critical event that is required not only for the development of skeletal muscle during embryogenesis but also for the regeneration of adult myofibers upon injury and for load-induced skeletal muscle hypertrophy. Augmenting the natural process of muscle cell fusion in existing or introduced myogenic cells has enormous therapeutic potential to treat degenerative muscle diseases. However, the mechanisms of myoblast fusion in vertebrates remain less understood. Accumulating evidence suggests that myoblast fusion in mammalian system involves the activation of specific cell signaling pathways such as MAPK, non-canonical NF-?B, canonical Wnt signaling. However, the upstream signaling mechanisms leading to their activation and how the activation of these pathways promotes myoblast fusion during myogenesis remain poorly understood. Our preliminary studies have identified that myeloid differentiation primary response gene 88 (Myd88) is a critical regulator of myoblast fusion both in vitro and in vivo. Our results show that the levels of Myd88 are increased during myogenesis. Genetic ablation of Myd88 impairs myoblast fusion whereas overexpression of Myd88 enhances the formation of myotubes in cultured myoblasts. Furthermore, we have obtained initial evidence that Myd88 regulates the activation of specific profusion signaling pathways during myogenesis. Based on our preliminary studies, we have proposed to establish the role and delineate the signaling and molecular mechanisms by which Myd88 mediates skeletal muscle formation in vitro and in vivo. Our working hypotheses are: (I) Myd88 promotes myoblast fusion in multiple conditions through augmenting the gene expression of specific profusion molecules; (II) Myd88 coordinates the activation of canonical Wnt and non-canonical NF-?B signaling to promote myoblast fusion during myogenesis; (III) Both transcriptional and post-transcriptional mechanisms regulate the levels of Myd88 in differentiating myoblasts. We will test these hypotheses by addressing the following three specific aims. Aim I: Establish the role and investigate molecular mechanisms by which Myd88 mediates myoblast fusion; Aim II: Investigate the signaling networks through which Myd88 promotes myoblast fusion; and Aim III: Investigate the mechanisms by which levels of Myd88 are increased during myogenesis. Successful completion of this project will provide critical insights into the mechanisms of myoblast fusion and will lead to the identification of novel drug targets for treatment of muscle disorders.